Particularly in the context of oil extraction systems, it is required to measure the proportions of the components in a multi-phase fluid flow, for example, a crude oil flow containing gas and/or water in varying proportions. It has been proposed in WO 90/13859 (FD20) to provide a mixer apparatus for fluids incorporating a choke or venturi, and to associate measuring or metering equipment with the venturi to determine mass flow rates of the fluid phases flowing through it. Such metering equipment can include a densitometer employing a .gamma. or X-ray source located at one side of the fluid flow for transmitting its radiation transversely through the fluid to a radiation sensor or detector located at the other side of the fluid flow.
The use of a .gamma. or X-ray source and detector presents a problem for such metering equipment in that the tubing within which the fluid flow is confined should be highly transparent to the metering radiation whilst being resistant to the fluid flow and having necessary strength and rigidity. The first of these requirements effectively precludes the use of steel tubing and it has been proposed in EP 0 236 623 to employ beryllium as the material of the wall of the tube across which the radiation is transmitted. The toxicity of beryllium however presents manufacturing problems.